Buried heterostructure devices with unique contact-facilitating layers

ABSTRACT

In the fabrication of buried heterostructure InP/InGaAsP lasers, mask undercutting during the mesa etching step is alleviated by a combination of steps which includes the epitaxial growth of a large bandgap InGaAsP cap layer (1.05 eV≲E g  ≲1.24 eV) and the plasma deposition of a SiO 2  etch masking layer. Alternatively, the cap layer may be a bilayer: an InGaAs layer or narrow bandgap InGaAsP (E g  ≲1.05 eV), which has low contact resistance, and a thin InP protective layer which reduces undercutting and which is removed after LPE regrowth is complete. In both cases, etching at a low temperature with agitation has been found advantageous.

This application is a divisional of copending application Ser. No.505,993, filed on June 20, 1983 U.S. Pat. No. 4,566,171.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor devices and, more particularly,to buried heterostructure lasers.

Semiconductor diode lasers fabricated from the InGaAsP/InP materialssystem are currently of interest for application in opticalcommunication systems operating at 1.0-1.6 μm. Of the wide variety ofpossible laser structures which can be fabricated, those which utilize areal refractive index waveguide, such as a buried heterostructure (BH),have recently been shown to have advantages with respect to the absenceof both self-pulsations in their outputs and kinks in theirlight-current behavior. [R. J. Nelson et al, Applied Physics Letters,37, 769 (1980).] For most refractive-index-guided laser structures, thefabrication sequence usually requires an etching step to form a mesawhich ultimately defines the lateral dimensions of the optical cavityand the active region of the laser. Since the transverse modecharacteristics of this cavity depend on, among other factors, thecavity geometry and dimensions, it is important to exercise a highdegree of control over these parameters during the etching process ifhigh yields of single transverse mode devices are to be obtained. Forexample, typically the dimensions of the active region of a BH laser,which operates in the fundamental transverse mode, are 0.15 μm inthickness and a maximum of 2.0 μm in width. Associated with the problemof strict geometrical and dimensional control is the problem of maskundercutting. This undercutting is often unpredictable, leading to aloss of dimensional and geometrical control over the desired mesas. Inthe case of a BH laser, having a 2.0 μm wide active layer, undercuttingof only 1.0 μm on each side of the mesa results in complete loss of themesas on the wafer. Even in cases where the mask undercutting ispredictable and can be allowed for, a large amount of mask overhang canlead to problems in later processing steps, such as LPE regrowth wherelocal growth dynamics can be adversely affected. It is clear, therefore,that the etchant system and etching technique used to fabricate mesasfor these laser structures must allow for precise geometrical anddimensional control with little or no undercutting. In addition, theetchant of choice should leave a contaminant-free surface which issmooth and free from pits or other defects which may lead to problems insubsequent processing steps.

Bromine-methanol is one etchant which has been utilized successfully asa preferential etchant in the fabrication of InGaAsP/InP BH lasers [M.Hirao et al, Journal of Applied Physics, 51, 4539 (1980); R. J. Nelsonet al, IEEE Journal of Quantum Electronics, 17, 202 (1981)] andburiedo-waveguide-heterostructure lasers [R. B. Wilson et al,International Electron Devices Meeting, Technical Digest, 370 (1980)].In those reports, however, emphasis was placed on device results ratherthan on the mesa etching process and the mask undercutting problem.

SUMMARY OF THE INVENTION

We have found that wet chemical etching can be used successfully tofabricate InP/InGaAsP BH lasers without significant undercuttingoccurring during the mesa etching step. In accordance with one aspect ofour invention, a large bandgap InGaAsP cap layer is epitaxially grown ontop of the BH and a SiO₂ masking layer is plasma deposited on the caplayer. The masking layer is patterned using conventionalphotolithographic techniques, and then mesas are etched preferably usinglow temperature Br-methanol with agitation. For limited undercutting andlow contact resistance the bandgap E_(g) of the cap layer should be 1.05eV≲E_(g) ≲1.24 eV. LPE regrowth of InP lateral confinement layers alongthe sides of the mesa completes the BH.

Another aspect of our invention contemplates the use of a multilayer caplayer including an InGaAs layer, or a narrow bandgap InGaAsP layer,which provides for low contact resistance, and a thin InP protectivelayer, which reduces undercutting and which is removed after LPEregrowth is complete. In this case LPE regrowth includes first growingthe usual InP confinement layers along the sides of the mesa and thengrowing a quaternary protective layer over the InP confinement layers sothat the InP protective layer can be selectively etched away withoutattacking the confinement layers.

BRIEF DESCRIPTION OF THE DRAWING

Our invention, together with its various features and advantages, can bereadily understood from the following, more detailed description takenin conjunction with the accompanying drawings, in which the figures havenot been drawn to scale in the interest of simplicity and clarity ofillustration:

FIG. 1 shows a heterostructure after mesa etching in accordance with oneaspect of our invention;

FIG. 2 shows a BH laser incorporating the structure of FIG. 1;

FIG. 3 shows another heterostructure after mesa etching in accordancewith a second aspect of our invention;

FIG. 4 shows the structure of FIG. 3 after regrowth of epitaxial layersto form a BH; and

FIG. 5 shows a BH laser incorporating the structure of FIG. 4.

Common elements in different figures have been given identical referencenumbers in order to facilitate comparison.

DETAILED DESCRIPTION

With reference now to FIG. 2, the buried heterostructure (BH) lasershown is fabricated by first epitaxially growing a doubleheterostructure (DH) wafer on a suitable single crystal substrate. Ingeneral, the epitaxial layers are then masked and etched to formelongated mesas, one of which is shown in end view in FIG. 1. Thegeometry of the mesa delineates the stripe shape of the active layerwhich is typically located near the neck of the mesa. Later, epitaxialregrowth of layers along the sides of the mesa surrounds the activelayers with wider bandgap, lower refractive index material and completeseach BH. Electrical contacts are applied to the top and bottom of thewafer, which is then diced, as by cleaving and sawing, into individuallaser chips. Finally, each laser is mounted on a suitable heat sink (notshown).

More specifically, consider the InP/InGaAsP BH laser of FIG. 2. Thislaser comprises an n-InP substrate 10 on which are epitaxially grown(illustratively by liquid phase epitaxy (LPE)) the following,essentially lattice-matched layers in the order recited: an n-InP firstcladding layer 12, an unintentionally doped In_(1-y) Ga_(y) As_(x)P_(1-x) active layer 14, a p-InP second cladding layer 16, and a p⁺-InGaAsP contact-facilitating cap layer 18. These layers form a doubleheterostructure (DH) wafer. The proportions x and y in the active layerare chosen according to the desired operating wavelength of the laser asdescribed, for example, by Olsen et al, IEEE Journal of QuantumElectronics, QE-17, 131 (1981).

In order to delineate elongated mesas of the type shown in FIG. 2 fromthis wafer, a SiO₂ etch-masking layer is deposited on cap layer 18 and,using standard photolithographic techniques, is patterned to form astripe mask 20 over each intended mesa. Etching with Br-methanoldelineates the mesa and narrows the active layer 14 to less than about2.0 μm wide. (It is typically about 0.1-0.2 μm thick.)

In accordance with our invention, the mesa is delineated withoutsignificantly undercutting the mask 20 by a combination of steps;namely, the cap layer 18 is made to have a bandgap E_(g) in the range1.05 eV≲E_(g) ≲1.24 eV, and the SiO₂ mask 20 is plasma deposited underparticular conditions (described more fully later). Then, the mesa isetched using Br-methanol at a low temperature (preferably about 0° C.)with agitation so as to increase the ratio of etch depth to undercut.Above the upper limit of E_(g), the contact resistance is undesirablyhigh, whereas below the lower limit excessive undercutting occurs.

Under these conditions, undercutting is no greater than about 0.5 μm ona side, and the specific contact resistance is less than 10⁻⁵ ohm-cm².

After etching the mesa structure shown in FIG. 1, InP layers 22 and 24are regrown by LPE along both sides of the mesa so as to surround theactive layer 14 with wider bandgap, lower refractive index material. Inaddition, the first layers regrown are p-InP layers 22 and the secondlayers are n-InP layers 24, thereby forming blocking p-n junctions 26therebetween. That is, when the cladding layers 12 and 16 areforward-biased to cause laser emission from active layer 14, theblocking junctions 26 are reverse-biased so that current is constrainedto flow primarily through the mesa and hence through the active layer.

Forward-bias voltage and pumping current are applied to the device via abroad area metal contact 28 formed on the substrate 10 and a stripegeometry metal contact 30 formed on n-InP layers 24. The stripe isdelineated by an opening in dielectric layer 32. The source of voltageand current is not shown.

In an alternative embodiment of our invention, a BH laser of the typedepicted in FIG. 5 is fabricated as follows. An InP/InGaAsP/InP DH(layers 12, 14, and 16) is fabricated in essentially the same manner asdescribed above. However, the cap layer 18' comprises narrow bandgap(E_(g) ≲1.05 eV) p⁺ -In GaAsP or p⁺ -InGaAs rather than the higherbandgap (E_(g) ≳1.05 eV) InGaAsP described above. In order to exploitthe superior electrical contacting properties of the narrow bandgapmaterial, a protective layer 19 of p-InP is grown over the cap layer18'. Layer 19 reduces undercutting and protects layer 18' duringsubsequent mesa etching steps. In order to define the mesa, SiO₂ stripes20 are plasma deposited onto the protective layer 19 as described above.Etching in Br-methanol results in the mesa configuration shown in FIG.3.

Next, LPE regrowth of InP layers along the sides of the mesa results inthe formation of blocking junctions 26 as previously described and asshown in FIG. 4. In addition, however, protective layers 25 of InGaAspare grown over the InP layers 24. The layers 25 enable the p-InPprotective layer 19 to be removed by a selective etchant such as 7M-12MHCl without also attacking the underlying InP blocking layers 22 and 24.The p-InP protective layer 19 is removed because it is difficult to makegood electrical contact to this material, whereas a far superiorelectrical contact can be made to the underlying narrow bandgap layer18'. The completed BH laser is shown in FIG. 5 where the broad areacontact 28 and the stripe contact 30 have been applied in the mannerdescribed in reference to FIG. 3.

EXAMPLE I

The following example describes experiments that were performed todemonstrate the superior undercutting characteristics of the combinationof a plasma deposited SiO₂ mask and an LPE-grown InGaASP cap layerhaving a bandgap E_(g) in the range of approximately 1.05-1.24 eV.

The substrate was Sn-doped (n≈10¹⁸ cm⁻³) InP and had a surfaceorientation within about 1° of the (001) {001}/{111} interfacial angleof 61°±2° was somewhat larger than the expected value of 54.74°. Thedifference may be due to effects of nearby vicinal planes, similar toresults found for Br-methanol etching of GaAs.

The mesa profile achieved using the etching techniques described herehas been used to fabricate BH lasers of the type shown in FIG. 2. A mesaof the particular geometry shown in FIG. 1 allows for a narrow activelayer stripe width (w) near the "neck" of the mesa, while maintaining awider electrical contact region (a) near the top of the mesa. Thegeometric relation between the neck width (w), the neck depth (d), andthe stripe width (a) is given by the expression:

    w=a-2d/ tan θ                                        (1)

where θ is the interfacial angle. Because the only depth parameter whichis easily measured is the total etch depth (h), the relationship betweenthe neck depth (d) and the total etch depth (h) must be determined inorder to utilize Eq. (1) in a practical fashion. The experimentaldetermination of this relation is described by the expression:

    d=0.61h+0.33.                                              (2)

Eq. (2), when combined with Eq. (1), yields the relation between (a),(w), and (h), which is:

    w=a-0.68h-0.37.                                            (3)

These expressions have been found to hold for multilayer InP/InGaAsPstructures (such as a DH) as well as for InP substrate material providedthat the same etching conditions (0° C. with agitation) are used andessentially no mask undercutting occurs. Assuming an accuracy of ±0.15μm in the measurement of the parameters (h) and (a), or (111) plane. Thedislocation density was determined from etch pitting studies to be about8×10⁴ cm⁻². The mesa-etching apparatus was a 100 ml beaker containingapproximately 80 ml of 1% (by volume) Br-methanol solution and aperforated Teflon™ basket for holding the wafer (Teflon is a trademarkof Dow Corning Corporation). Although both plasma deposited SiO₂ and Si₃N₄ etching masks were tried, we found that SiO₂ masks deposited underthe following conditions were preferred from an undercutting standpoint.

A commercially available plasma deposition system (Plasma Therm PK-12)was used. The measured plasma RF power density was about 40-50 mW/cm²,the chamber pressure was about 500-1000 mTorr, and the substrate tabletemperature was about 200°-300° C. When gas concentrations of 3% silanein argon (324 sccm) and 100% nitrous oxide (420 sccm) were mixed in thechamber, the deposition rate was 670 Å/min. The resultant SiO₂ films hada refractive index of 1.47±0.015, an etch rate in BOE (6:1, NH₄ F:HF) of3200 Å/min., and low compressive stress of about 1×10⁹ dynes/cm. TheseSiO₂ films were also found to produce less undercutting than SiO₂ filmsdeposited using other techniques such as sputtering.

Using this plasma deposition procedure, 3000 Å of SiO₂ was deposited onthe (001) surface of the wafer. Stripes and windows were then definedalong each orientation ([110] and [111]) using standardphotolithographic techniques. These samples were then etched to a depthof 4.0-5.0 μm using a 1% (by volume) Br-methanol solution at anessentially constant temperature of about 0° C. The resultant mesa andchannel profiles were observed from the appropriate cleavage plane.Because crystallographic planes near the {111}A planes tend to etch theslowest, these planes tend to develop as etching proceeds. For thisreason the planar crystallographic features found are thought to beclosely related to {111}A planes, although the measured the calculatedaccuracy in the determination of the parameters (d) and (w) from Eqs.(2) and (3) is ±0.10 μm and ±0.18 μm, respectively.

In the development of Br-methanol etching for channel and mesaformation, we have paid considerable attention to the problem of maskundercutting. In the fabrication of BH lasers, mask undercutting canresult in the loss of precise dimensional control over the mesa, leadingto a poor yield of single transverse mode devices. In addition, maskundercutting is frequently nonuniform (along a BH stripe, for example),and large scattering effects may result from the widely varying lateraldimension of the active region. Finally, excessive mask undercutting mayseriously influence the manner in which epitaxial layer growth takesplace near a mesa or channel in a subsequent regrowth step and is,therefore, again undesirable.

Two parameters which influence mask undercutting during the etchingprocess are temperature and sample/solution agitation. SEM micrographswere taken of mesas etched to the same depth using a 1% Br-methanolsolution at 0° C. (with agitation), and 25° C. (with minimal agitation),respectively. The etching mask used for these samples was plasmadeposited SiO₂ as described above. Qualitatively, we found that in thecase of the mesa etched at 25° C. with minimal agitation the sidewallstend to be significantly rounded, with a weak (111)A crystallographicfeature and a total-etch-depth:undercut ratio of about 2:1. In contrast,the mesa etched at 0° C. with agitation was characterized by a strong(111)A crystallographic feature and a total-etch-depth:undercut ratio inexcess of 20:1.

A third factor which has been found to influence undercutting is maskcomposition. As mentioned previously, we found that plasma deposited Si₃N₄ etching masks tend to undercut more than plasma deposited SiO₂ masksunder identical etching conditions. This difference may be related to astress-induced enhancement of the etch rate near the mask, since we havefound a weak correlation between the degree of undercutting and the Si₃N₄ deposition parameters. However, effects due to mask adhesion orsurface-enhanced diffusion of the etchant cannot be ruled out.

An additional factor which has been found to influence mask undercuttingduring the fabrication of BH lasers is the composition of the top p⁺-InGaAsp cap layer normally grown on the DH for contact purposes. Wefind that a p⁺ -InGaAsP layer having a bandgap of 0.97 eV(lattice-matched to InP) tends to undercut significantly, whereas a p⁺-InGaAsP having a bandgap of about 1.20 eV essentially eliminates thisproblem. More specifically, for undercutting of less than about 0.5 μmon a side, the bandgap of the cap layer should be greater than about1.05 eV, but for a specific contact resistance of less than about 10⁻⁵ohm-cm the bandgap should be less than about 1.24 eV.

EXAMPLE II

The following example describes the use of the procedures of EXAMPLE Ito fabricate a BH laser of the type shown in FIG. 2.

On a (100)-oriented, Sn-doped, InP substrate (n⁻ 10¹⁸ cm⁻³) we used LPEto grow the following essentially lattice-matched, epitaxial layers inthe order recited: a 6 μm thick Sn-doped n-InP (n.sup.˜ 2×10¹⁸ cm⁻³)cladding layer 12, a 0.2 μm thick, unintentionally doped In_(x) Ga_(1-x)As_(y) P_(1-y) (x=0.75, y=0.55, E_(g) =0.99 eV) active layer 14, a 2.6μm thick Zn-doped InP (p.sup.˜ 1×10¹⁸ cm⁻³) cladding layer 16, and a 0.7μm thick Zn-doped InGaAsP (p.sup.˜ 4×10¹⁸ cm⁻³, E_(g) =1.20) cap layer18.

To form mesas from this heterostructure, a 0.3 μm thick SiO₂ layer wasplasma deposited on cap layer 18 and was patterned using standardphotolithography to define stripe masks 20 which extended along the[110] direction. A solution of 1% Br in methanol was used to form themesa of FIG. 1 with h=5.3 μm and a=5.6 μm, from which we used Eqs.(1)-(3) to calculate that w=1.63 μm and d=3.56 μm. Undercutting of mask20 was measured to be less than 0.2 μm on a side.

Next, LPE was again used to regrow a 1.0 μm thick, Zn-doped (p.sup.˜8×10¹⁷ cm⁻³) InP blocking layer 22 and a 4.5 μm thick Sn-doped (n.sup.˜2×10¹⁷ cm⁻³) InP blocking layer 24, both along the sides of the mesa sothat the top of layer 24 was essentially co-planar with the top of themesa.

Standard evaporation was used to deposit a Au/Sn/Au broad area contact28 on substrate 10 and to deposit a Au/Zn stripe geometry contact 30 onthe cap layer 18. Contact 30 was delineated by an opening in dielectric(e.g., SiO₂) layer 32.

After metallization was complete, the wafer was diced into laser chipsusing standard sawing and cleaving. In operation, these lasers exhibiteda median room temperature c.w. threshold current of 28 mA. Oscillationoccurred both in the fundamental transverse mode and in a singlelongitudinal mode at λ=1.32 μm.

EXAMPLE III

This example describes experiments which demonstrate the efficacy ofusing a narrow bandgap InGaAsP cap layer to facilitate contacting withan InP protective layer to limit undercutting.

Once again (100)-oriented, Sn-doped n.sup.˜ 10¹⁸ cm⁻³) InP substrateswere employed. A 0.7 μm thick, Zn-doped (p.sup.˜ 1×10¹⁹ cm⁻³) InGaAsP(λ=1.55 μm) layer was LPE-grown on the substrate, and a 0.25 μm thick,Zn-doped (p.sup.˜ 2.5×10¹⁸ cm⁻³) InP layer was grown on the InGaAsPlayer. As described in EXAMPLE I, plasma deposited SiO₂ mask stripeswere formed along the [110] direction and the mesas were etched to adepth of 6.2 μm using Br-methanol. The InP layer limited undercutting ofthe mask to less than 0.1 μm on a side.

Thus, this procedure followed by the regrowth and selective etchingsteps described with reference to FIGS. 3-5 can be used to fabricate aBH laser of the type shown in FIG. 5.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specification embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, while the variousaspects of our invention have been described in terms of light emittingdiodes operating as lasers, it will be apparent that the invention isalso applicable to light emitting diodes operating as sources ofspontaneous emission (e.g., edge-emitting LEDs).

What is claimed is:
 1. A buried heterostructure device comprising:anelongated, mesa-shaped, double heterostructure which includes a pair ofopposite-conductivity-type InP cladding layers and an InGaAsP activelayer disposed between said cladding layers and essentiallylattice-matched thereto, a p⁺ -InGaAsP contact-facilitating layer formedon said heterostructure and essentially lattice-matched thereto, and apair of opposite-conductivity-type InP blocking layers on each side ofsaid mesa so as to form therebetween a p-n junction which isreverse-biased when said cladding layers are forward-biased,characterized in that said contact-facilitating layer has a bandgap inthe range of approximately 1.05-1.24 eV.
 2. The device of claim 1 foruse as a laser and further including electrode means for forward-biasingsaid cladding layers and applying current to said active layer in excessof the lasing threshold, thereby to generate optical radiationtherefrom, and resonator means for causing stimulated emission of saidradiation.
 3. A buried heterostructure semiconductor devicecomprising:an elongated, mesa-shaped, double heterostructure whichincludes a pair of opposite-conductivity-type InP cladding layers and anInGaAsP active layer disposed therebetween and essentiallylattice-matched thereto, a p⁺ -type contact-facilitating layer on saidheterostructure and essentially lattice-matched thereto, andcontact-facilitating layer comprising a compound which includes In, Gaand As and has a bandgap less than about 1.05 eV, a pair ofopposite-conductivity-type InP blocking layers epitaxially grown alongeach side of said mesa so as to form therebetween a p-n junction whichis reverse-biased when said cladding layers are forward-biased, and anInGaAsP layer formed on top of each of said pairs and adjacent saidcontact-facilitating layer.
 4. The device of claim 3 wherein saidcontact-facilitating layer comprises a ternary compound of InGaAs. 5.The device of claim 3 wherein said contact-facilitating layer comprisesa quaternary compound in InGaAsP.
 6. The device of claim 3 for use as alaser and further including electrode means for forward-biasing saidcladding layers and applying current to said active layer in excess ofthe lasing threshold, thereby to generate optical radiation therefrom,and resonator means for causing stimulated emission of said radiation.7. The device of claim 3 including a single crystal n-InP substrate onwhich said heterostructure is formed and wherein said electrode meanscomprises at least one metal layer making ohmic contact to saidcontact-facilitating layer and at least one other metal layer makingohmic contact to said substrate.